1. Field of Inventions
The present invention relates generally to medical devices that support one or more diagnostic and/or therapeutic elements in contact with body tissue associated with, for example, bodily orifices or lumens.
2. Description of the Related Art
There are many instances where diagnostic and/or therapeutic elements must be inserted into the body. For example, therapeutic elements may be used to form lesions to treat conditions in the heart, prostate, liver, brain, gall bladder, uterus, breasts, lungs and other solid organs. There are also many ways to form lesions. The application of electromagnetic radio frequency (“RF”) energy to heat and eventually kill (i.e. “ablate”) tissue is one method of forming a lesion. During the ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), tissue coagulation occurs and it is the coagulation that kills the tissue. Thus, references to the ablation of soft tissue are necessarily references to soft tissue coagulation. “Tissue coagulation” is the process of cross-linking proteins in tissue to cause the tissue to jell. In soft tissue, it is the fluid within the tissue cell membranes that jells to kill the cells, thereby killing the tissue. The tissue coagulation energy is typically supplied and controlled by an electrosurgical unit (“ESU”) during the therapeutic procedure. More specifically, after an electrophysiology or electrosurgical device has been connected to the ESU, and the electrodes or other energy transmission elements on the device have been positioned adjacent to the target tissue, energy from the ESU is transmitted through the electrodes to the tissue to from a lesion. The amount of power required to coagulate tissue ranges from 5 to 150 W.
With respect to the formation of therapeutic lesions in the heart to treat cardiac conditions such as atrial fibrillation, atrial flutter and arrhythmia, some procedures utilize catheters which form lesions on the endocardium in order to create a maze for electrical conduction similar to that created by surgical maze procedures. The lesions are carefully placed to interrupt the conduction routes of the most common reentry circuits.
Lesions within the heart may be formed by ablating tissue with one or more electrodes carried by a catheter. Catheters used to create lesions typically include a relatively long and relatively flexible shaft that carries the electrodes at or near its distal end. The proximal end of the catheter shaft is connected to a handle which may or may not include steering controls for manipulating the distal portion of the catheter shaft. The length and flexibility of the catheter shaft allow the catheter to be inserted into a main vein or artery (typically the femoral artery), directed into the interior of the heart where the electrodes contact the tissue that is to be ablated. Fluoroscopic imaging is used to provide the physician with a visual indication of the location of the catheter. Exemplary catheters are disclosed in U.S. Pat. Nos. 6,013,052, 6,203,525, 6,214,002 and 6,241,754.
More recently, surgical soft tissue coagulation probes that carry one or more electrodes on relatively short, stiff shafts have been developed. These probes may be used in endocardial and epicardial procedures where access to the heart is obtained by way of a thoracostomy, thoracotomy or median sternotomy. Such probes also allow endocardial lesions to be formed as a secondary procedure during a primary open heart surgical procedure such as mitral valve replacement, aortic valve replacement, and coronary artery bypass grafting. Exemplary surgical probes are disclosed in U.S. Pat. No. 6,142,994, U.S. Pat. No. 6,468,272 and U.S. Pat. No. 6,645,200.
One lesion that has proven to be difficult to form with conventional catheters and surgical probes (collectively referred to herein as “probes”) is the circumferential lesion that is used to isolate a pulmonary vein and cure ectopic atrial fibrillation. Lesions that isolate a pulmonary vein may be formed in or around the pulmonary vein ostium or within the pulmonary vein itself. In some instances, these circumferential lesions are formed by dragging a tip electrode around the pulmonary vein or by creating a group of interconnected curvilinear lesions one-by-one around the pulmonary vein, both of which can be slow and/or leave gaps of conductive tissue after the procedure. It can also be difficult to achieve the adequate tissue contact with conventional catheters. In other instances, inflatable balloon-like devices that can be expanded within or adjacent to the pulmonary vein have been introduced. Although the balloon-like devices are generally useful for creating circumferential lesions, the inventors herein have determined that these devices have the undesirable effect of occluding blood flow through the pulmonary vein.
More recently, it has been proposed to provide probes with a pre-sized, continuous distal loop that positions a ring of electrodes transverse to probe shaft and around the target tissue structure when deployed. The present inventor has, however, determined that achieving the desired level of tissue contact with such a continuous loop can be dependent on the physician having accurate information concerning the size of the target structure so that the properly sized loop will be deployed. If the loop is too small, there will be insufficient tissue contact and, if the loop is too big, there is a risk of damage to the target structure.